He may have been showered with accusations of electioneering from opposition parties and a mixed bag of reactions from the unions, but Gordon Brown managed to get the thumbs up from Bono and Bob Geldof for including a "wonderful thing" in his last batch of bills before the general election – the enshrinement in law of a funding promise to the world's poorest people.

International aid campaigners welcomed the prime minister's decision to include in his legislative agenda the international development spending draft bill, which will put the government's commitment to spend 0.7% of national income on development from 2013 on to the statute books.

But opposition parties accused the prime minister of watering down a promise to enshrine the commitment in law by reducing the bill to draft status only.

Bono, the lead singer of U2 and co-founder of the campaign group One, hailed the decision and urged parliament to ensure the bill is pushed through.

"The proposal to make the 0.7% pledge legally binding is not just a great announcement, it is transformative of real lives, by a government that has led the world in keeping its promises to the world's poorest people," Bono said. "The next step is making sure this becomes law as soon as possible, in 2010."

Fellow One campaigner Geldof said the legislation could be "a rare but wonderful thing" if political parties allow the bill to go through.

"The gains African countries have made over the past decade are under threat from two crises not of their making: global recession and climate change. It's good to see the British government taking steps to mitigate the impacts of these predicaments, which is why it is important that this legislation is enacted sooner rather than later."

In principle such a bill should have little problem being pushed through as the Conservative party has also pledged to meet the UN target of 0.7% of national income spent on international aid, though a spokesman for Douglas Alexander, the international development secretary, claimed that the Tories would not back the draft bill.

"Andrew Mitchell has repeatedly refused to back legislation to enshrine our commitment to spend 0.7% of national income on international development from 2013," he said. "Only Labour has committed to legislate for this promise."

However, a spokesman for the Tories said the party "would support a bill on 0.7% but clearly the government have stopped well short of doing this themselves".

The Liberal Democrats pointed to the bill's draft status, which reduces its chances of becoming law in this session.

Michael Moore, the Lib Dems' spokesman for international development, accused Brown of stepping back from a promise made at the Labour conference in September to push this piece of legislation through.

"Gordon Brown made a firm commitment to enshrine the 0.7% target for aid spending in statute, but just seven weeks on he has abandoned that promise," Moore said.

"With a general election only months away, and the Tories' commitment to development issues far from certain, this Queen's speech is nothing but another broken promise to the world's poorest people."